1. Field of the Invention
The present invention relates to a system for measuring a parameter such as steam quality and to a mixer for mixing liquid and gaseous vapor in order to permit accurate measurement of steam quality.
2. Description of the Prior Art
Steam quality is a parameter used for many purposes in a wide variety of industries, including power generation, oil recovery, freeze protection, process heating, and the like. The mass ratio of dry steam to the total fluid mass within a steam sample is termed steam quality and is usually expressed as a percentage. The energy content of wet steam is dependent upon its "steam quality," and for this reason it is important to measure steam quality to determine the energy in the steam, which in turn determines the steam's ability to achieve a desired heating result. If steam quality is known, the rate of energy or power available from the steam to accomplish a particular job is capable of being determined.
In particular, in oil recovery systems, it has been known that injecting steam into wells drilled in the oil field will make the crude oil more flowable and will increase the output of the coil field. The measurement of the steam quality at particular locations immediately before being injected into a well head permits the operators to maximize the efficiency of the steam heating of the oil field, and in this way aid in efficient operation of the oil pumping system.
Particularly in oil fields, the steam sometimes is carried miles from the steam source, and during this travel time some condensation occurs. The liquid phase or droplets in the steam can be moving at a different flow rate than the vapor portion of the steam, creating difficulties in measuring the density of the steam. In particular, there can be condensed liquid moving along the bottom of a pipe.
Density measuring instruments using radiation have been used on steam, but with less than satisfactory results when the fluid is wet steam. Variations in the make-up of the steam such as different flow rates of the liquid and vapor phases affect the densitometer's output and thus conditioning of the steam is desired. Simple orifice plates have been used in an attempt to condition steam for density measurement, however results with wet steam have been unsatisfactory. In order to obtain steam quality information, the pressure and the density of the steam must be determined. Steam quality is determined according to the general equation: EQU Steam Quality=(Vs-Vl)/(Vv-Vl)
Where:
Vs=Specific volume of the steam. PA1 Vl=Specific volume of the liquid portion. PA1 Vv=Specific volume of the vapor portion.
The present invention solves the problems associated with ensuring that the liquid phase and the vapor phase of the steam are sufficiently homogenized so that sufficiently accurate density measurements can be obtained to permit an accurate calculation of steam quality.
An article entitled "Measurement of Steam Quality, Mass Flow Rate, and Enthalpy Delivery Rate using combined Neutron Densitometer and Nozzle," by G. E. Woiceshyn et al., was presented at the Society of Petroleum Engineers/Department of Energy, 5th Symposium on Enhanced Oil Recovery, held in Tulsa, Okla. in April, 1986, and published as paper SPE/DOE/4907, and is included herein by reference. This paper discusses the need for accurate measurement of steam quality at the well head for efficient operation and reliable evaluation of the recovery projects that utilize steam injection. This publication shows the use of neutron density detectors and associated equations for determining steam quality as well as other steam parameters.
Proper mixing in the region where density measurement is made is extremely important in solving the problems of varying flow rates and obtaining proper measurements of density over a wide range of flow rates at a fixed density measurement "window" or location on the pipe. In order to obtain proper mixing, it is desirable to have the steam flowing in a velocity range where the liquid phase can be well mixed with the vapor phase. Velocity can be increased by using a simple orifice plate in the steam pipe, but this technique can provide mixing over only a limited range of flow velocities, and the location downstream from the orifice where the proper mixture is obtained can vary undesirably with the flow rate. The orifice plate also has the disadvantage of creating a large pressure drop which, particularly on large lines, results in a large energy loss. Commercially available vane type mixers can reduce the pressure drop, but do not mix adequately over a normal range of flow velocities. These vane type mixers also have a substantial surface area against which water can collect, tending to minimize the mixing ability of the vane type mixer.
The effects of velocity of steam in the pipe are so varied or inconsistent that obtaining a homogeneous mixture of the two-phase flow (liquid and vapor) through a wide enough pressure and flow range to make a desired measurement with existing mixers is riddled with problems. Water can drop out, water can collect on the pipe walls, and there can be stratified flow at different parts of the pipe, and all of these problems can affect the measurements of density and steam quality. Thus the problems are substantial and a low cost, low pressure drop mixer that breaks up the water particles into small enough droplets to be carried at a reasonable velocity and which does not have the problems of separating water out of the steam flow before the density measurement is made is needed.